The present invention relates to an impact-echo analyzing apparatus and method for use in the field.
Many different methods are currently available for the non-destructive testing of materials, amongst which are: acoustic impact, infrared thermography, and ground penetrating radar. Although each of these methods is a viable technique for detecting faults and defects in materials, each has drawbacks that prevents its universal application. This is most particularly true with respect to availability in the field. No existing method provides reliability under various site conditions, nor provides the capability for rapid on-site testing of large areas. Even worse, all of the present techniques for non-destructive testing are complex and cumbersome, and are not easily applied.
The most serious problem affecting the service life of reinforced concrete structures is the formation of delaminations due to disruptive stresses. These disruptive stresses are caused by the weathering and corrosive deterioration of internal steel reinforcements.
Recently, the method of impact-echo testing of concrete slabs has been shown to provide reliable results in the detection of voids, defects and delaminations. This technique was also demonstrated to be capable of detecting delaminations in concrete slabs having asphalt overlays.
It is one thing, however, to provide a laboratory technique for analyzing concrete specimens, and another to provide the capability of performing such a test in the field. In the laboratory, the impact-echo technique produces a time-domain displacement waveform, which is then converted to an amplitude versus frequency spectrum. The frequencies obtained are indicative of the thickness of the plate and/or the depth of the internal crack or defect. It often requires an expert to interpret displacement waveforms, but amplitude spectra can be interpreted by a trained engineer. The two difficulties in converting the laboratory technique to a practical field technique are the need for: (1) a rugged, and easy to use instrument, and (2) a means for automating the signal analysis, particularly in applications requiring immediate, on site results.
The invention has developed a method of impact-echo testing of slab-like structural elements in the field. The method can be used in the field in the testing of: walls, pavements, and bridge decks. The inventive method exploits the discovery that by replacing the amplitude/frequency spectrum with a non-dimensional reflection spectrum, a more instantaneous understanding and analysis of defects in plate-like structures is achieved. The non-dimensional reflection spectrum is obtained by converting the frequency data from a fast Fourier transform of the displacement waveform, to non-dimensional depth data expressed as a percentage of the thickness according to the following equation: EQU D=C.sub.p /2f (1)
where:
f is the frequency of the P wave; PA1 C.sub.p is the P wave speed; and PA1 D is the thickness of the test specimen or the depth of a defect; PA1 Normalizing the depth "D" by dividing by the full thickness "T" of the structure gives the scale for the reflection spectrum.
In accordance with the invention, a neural network system has been developed for storing, analyzing, and classifying reflection spectra from other impact-echo tests on plate-like structures, such as walls, slabs, and bridge decks. The comparison of the test data with both stored flaw-free and flaw-containing data provides an instantaneous analysis of the test data at the test site. The neural network analysis developed for slab-like structures provides the test operator with information that: (1) the probability of the existence of a defect, and (2) the probable depth of the detected defect. The analysis and the data storage and retrieval can be accomplished using a portable computer so that on-site inspection and evaluation becomes a reality.
In addition, a new hand-held impact testing device has been developed that operatively connects to a portable computer programmed to process the test data. Data can be processed automatically using the neural network system, or it can be analyzed by an operator. The hand-held impact tester comprises a novel triggering apparatus, wherein different spherical impactors can be selected and released to impact a targeted test specimen. The impactors provide the proper duration of impact, whereby stress waves propagate through the test structure and reflect back from the external slab surfaces and/or internal defects in the slab. With the proper impact, voids and delaminations can be ascertained with a high probability, and the depth of the defect can be determined with precision.
The new impact tester also provides a novel sequence of testing events wherein a highly sensitive transducer is brought into contact with the work surface and electrically activated slightly prior to the contact of the spherical impactor with the test surface. Thereafter, the impact mechanism is designed to introduce stress (sound) waves into the test structure. The transducer receives the reflected waves and converts the waves into an electrical signal for processing purposes. The transducer is thereafter electrically deactivated and retracted from the test surface by the impact mechanism for return to its disarmed position.
The new impact testing device is a small, self-contained, hand-held unit. The built-in testing sequence of the self-contained testing unit provides the rapid speed that until now has never been achieved in the field.